Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
Optical encoders utilize a light source and a photodetector to measure changes in the position of an encoding disk or strip. In a transmissive encoder, the encoding disk includes a series of alternating opaque and transparent strips. The light source is located on one side of the code strip, and the photodetector is located on the other side of the code strip. The light source and photodetector are fixed relative to one another, and the code strip moves between the photodetector and the light source such that the opaque regions of the code strip interrupt the light reaching the photodetector. The position of the code strip is determined by measuring the transitions between the light and dark regions observed by the photodiode.
In a reflective encoder, the light source and photodetector are located on the same side of the encoding strip, and the encoding strip consists of alternating reflective and absorbing stripes. The light source is positioned such that light from the light source is imaged into the detector when the light is reflected from the reflective strips.
Transmissive encoders have a number of advantages over reflective encoders in terms of tolerance and contrast ratios. In a reflective encoder, the distance between the code strip and the detector is critical as either the code strip itself or the light source as seen in the reflected light from the code strip is imaged into the detector. Hence, if there is an error in the code strip to detector distance, the image will be out of focus and errors will result.
In a transmissive encoder, the light from the light source is colliminated before it reaches the code strip, and hence, the light leaving the code strip is also colliminated. The detection assembly needs only to image this colliminated light onto the detector surface. Hence, the only critical distance is the distance from the imaging lens to the detector, which can be tightly controlled by the detector manufacturer independent of the specific encoder assembly.
Unfortunately, transmissive recorders require that two separate components, the light source and photodetector, be mounted and aligned with one another at the time of assembly of the encoder. Reflective encoders, in contrast, are constructed from a single emitter-detector element that is packaged together with the various optical components for imaging the light source onto the photodetector. This reduces the cost of assembly. In addition, there is a trend toward smaller and smaller mechanical systems. As the size of the mechanical systems in which the encoders are used is reduced, the problems associated with mounting components on both sides of the code strip become more severe. In such systems, reflective encoders have significant advantages in spite of the problems discussed above.
Reflective encoders, however, have significantly worse signal-to-noise ratios due to the internal reflection of the light source within the source-detector module. In a reflective encoder, the light source and detector are encapsulated together in a transparent material that also provides the lens functions needed to illuminate the code wheel in the desired manner and to image the light onto the detector. Part of the light generated by the light source is reflected at the encapsulation-air boundary back toward the detector. This light forms a background that is independent of the code wheel, and hence, lowers the signal-to-noise ratio of the encoder.